Most people do not use CMYK much in Photoshop. CMYK is much more useful in a tool like Adobe Illustrator. It is a nice feature but for a pretty niche segment.
AFAIK, anything to be printed requires CMYK support for decent results --- otherwise there's no control over printed colors. Printing might have gone out of fashion in recent years, but for the better part of the two decades it hadn't been the case.
Most companies/consumers only need professionally printed stuff like that rarely though, and when they do they go to see a printing professional who will do the necessary conversions for them.
For web use, app use, or other display-based use, RGB (and related) is usually enough, and that’s what people have been using the GIMP for.
